1. Field of the Invention
The present invention is directed to a computer tomography apparatus of the type having a partial ring x-ray source, over which a focus is electronically moved to generate an x-ray beam for transilluminating a measurement field from different directions, and a partial ring detector in the form of a row of detector elements which, in combination, generate a plurality of data sets from the respective directions, from which an image can be reconstructed in a computer.
2. Description of the Prior Art
In conventional computer tomography systems employing a partial ring x-ray source (radius R.sub.F and arc R.sub.F) and a partial ring detector (radius R.sub.D and arc R.sub.D), the sinogram, i.e., the arrangement of the detector data sets in a matrix, is only partially filled.
In order to obtain a complete image, therefore, some type of interpolation is necessary.
This problem is caused by the use of the partial rings, and is independent of the specific manner by which the focal spot is generated (for example, deflected electron beam, surface heating with a deflected laser beam, or revolving tube), is independent of the type of measurement (for example, using individual detectors), and is independent of geometrical factors with respect to the z-direction such as, for example, both partial rings being disposed in the same plane (which can be achieved by nutation of the detector partial ring), the partial rings being offset in z-direction, continuous advancing of the patient support during the measurement (spiral CT), and various combinations of these factors.
For explaining the problem, FIG. 1 shows an exemplary apparatus having an anode and a detector ring which are offset in the z-direction, and a patient support which is displaceable (advanceable) during the exposure. Further details of the operation of this type of system are described below.
The manner by which partial fan beams of variable size arise as result of the partial rings, in addition to full fan beams, i.e., fan beams which entirely cover the examination region, is shown in FIGS. 2 and 3. The measured values are usually characterized by two angle variables, with alpha being the angular position of the fan beam center (which is also the center of the detector ring), and beta being the angular position within the fan beam. The matrix of the data compiled in an alpha-beta coordinate system is referred to as the sinogram, with an exemplary sinogram being shown in FIG. 3. In the exemplary apparatus of FIG. 2, the detector is referenced 7, the anode is referenced 3, and the measurement field is referenced 6.
Measured values which correspond to a transillumination of the subject in identical paths (straight lines), but in an opposite propagation direction, are referred to as complementary. FIGS. 4 through 7 respectively illustrate the definition of the measured values and their classification as shown in FIG. 3. In FIG. 3, the region designated (0) indicates that data for those alpha-beta coordinate values are lacking, i.e., either a detector section or a focal path section corresponding to those angles is not present. The region of the matrix designated (1) means that single data points are present, i.e., a detection value is present but a complementary value is lacking. The regions designated (2) and (2*) indicate double values are present, i.e., data point and it complementary value are both present, respectively contained in the regions (2) and (2*).
The device geometry is referred to as "minimal" when at least one measured value is present for every point of the examination region (the central circle in FIG. 2) and for every direction. In systems having rings of smaller circumferential extent, this property may not be present.
The angles for measuring a ray AB (used as an example) are schematically shown in FIG. 4. The angles for measuring the complementary ray A'B' are schematically shown in FIG. 5. The conditions for a minimal geometry are schematically indicated in FIG. 6 where R.sub.o designates radius of measurement field. FIG. 7 shows the manner by which the respective rays are classified, consistent with the sinogram occupation designations shown in FIG. 3, for the minimal partial ring geometry of FIG. 2.
A technique for image reconstruction on the basis of weighting of the data of the complementary areas (2) and (2*) of a sinogram obtained in a CT apparatus of the fourth generation, having a full 360.degree. anode ring and a minimal detector ring, is described in the article "Optimal Short Scan convolution Reconstructions for Fanbeam CT," Parker, Med. Phys. 9(2), March/April 1982, pp. 254-257.
If both rings are only partial rings, the usual image construction technique requires that "artificial" measured data be created by interpolation into the complementary data, in order to convert the sinogram into the type which can be used in the so-called "Parker weighting" exemplified by the above article, wherein a hypothetical, complete anode ring is postulated.
Such complementary interpolation has the following disadvantages. First, data inconsistency exists. This is because for physical and technical reasons, the measured values from the opposite directions do not precisely coincide, since the fan beams from the opposite directions cannot be precisely the same. The inconsistency of neighboring measured values within a fan beam is further intensified by the convolution, and easily results in artifacts in the image. This disadvantage is generally attempted to be countered by undertaking a mathematical data smoothing, however, any such smoothing results in some information loss. A second disadvantage is that, except in the case of very specific angle relationships, an interpolation is always necessary, which can generate additional artifacts. A third disadvantage is that in systems of the type wherein the anode and detector rings are offset in z-direction, and/or wherein the patient support is advanced during the measurement, the inconsistency of the data is drastically intensified. A fourth disadvantage is the complex and expensive hardware outlay which is necessary. Such hardware is necessary because the processing of individual fan beams independently of each other, which is standard in conventional pre-processing and convolution (such as according to the pipeline principle in which data can be processed in parallel) is interrupted, because input data from different fan beams are required for augmenting a data set with the artificially created values.